320
A.MJ. Meijerink
drainage. Fractures and lineaments are obliterated by the weathered zone. (2)
along the strike of the rocks (gneisses) of zone 1 to the north, there is a zone which
is influenced by renewed erosion. Soils are shallow, slopes are short and steep,
thus runoff is much and fast, and recharge is little or absent. However, the set of
processes in this hydro tope produced narrow sandy alluvium which is locally a
good aquifer, recharged by bank storage and transmission losses. (3) The forested
hills. This is the only zone where during 2 months there is a rainfall excess over
evapotranspiration because of orographic precipitation. The few raingauges in the
area are only in the lower parts where the annual rainfall varies from 400 to 700
mm. The forested hills - no gauge- must receive over 1100 mm., a good example
of updating isohyetal maps by remotely sensed imagery. (see v.d. Laan, 1986;
Meijerink et aI., 1994). (4) The area with fairly thick colluvial and weathered zones
with low savannah vegetation, local recharge and stream recharge from the
adjoining hills. (5) Thin, young lavas resting on the basement, little recharge and
storage of groundwater.
Photo-lineaments can be described as; Linear structural elements which are
thought to have developed over fracture zones, and which are visible on remotely
sensed imagery. Some of them can be seen on the Colour Plate 14.B.
Interpreted lineaments can pertain to fractures of a different tectonic nature, with
or without intrusives or secondary clay fillings. Low topographic corridors of some
straightness, formed by denudation initiated by open fracturing, have been
identified as lineaments, as well as sharp, linear features in outcrop areas. There is
general agreement that the most promising water bearing directions originate from
brittle deformation caused by tensional stress related to normal faulting and strikeslip faulting (Larson, 1984; Greenbaum, 1 986; Castaing et al.,1989; Du Wenchai
and Ye Deliao, 1993). Different rocks respond to the same overall stress field with
different fracture densities. Grouping such rocks as gneisses which do not fracture
easily, together with granites which have the opposite behaviour, will therefore
cause much unnecessary variation in the statistical analysis.
The simple global relationship of well yield - distance to lineament may be of
little diagnostic significance. Indeed, the results of studies contradict each other
(see Waters, 1990, for a review and discussion). First of all, a segmentation
according to directions should be made, and specific capacity (l/hr/m.drawdown)
rather than yield (l/hr) should be used, if data permit.
The information of conventional rose diagrams can be supplemented, as Briere
and Razack (1982) have shown, by directional variograms and by relating the
results to the tectonic history of the area (e.g., Djeuda Tcapnga and Ekodek,
1990). Furthermore, the hydrogeologic situation around lineaments should be
considered, because much water may be derived from· weathered zones, with
possibly appreciable specific yields, adjacent to lineaments.
Because of their varied nature the hydrogeologic significance of a lineament
remains to be proven by drilling and well testing. (Carruthers et al. 1993).
In carbonate rocks (limestones, dolomites) lineaments are delineated in the
hopeful expectation that they represent zones of weakness extending sufficiently in
depth to have caused widening by solution by groundwater circulation. With the
A.MJ. Meijerink
drainage. Fractures and lineaments are obliterated by the weathered zone. (2)
along the strike of the rocks (gneisses) of zone 1 to the north, there is a zone which
is influenced by renewed erosion. Soils are shallow, slopes are short and steep,
thus runoff is much and fast, and recharge is little or absent. However, the set of
processes in this hydro tope produced narrow sandy alluvium which is locally a
good aquifer, recharged by bank storage and transmission losses. (3) The forested
hills. This is the only zone where during 2 months there is a rainfall excess over
evapotranspiration because of orographic precipitation. The few raingauges in the
area are only in the lower parts where the annual rainfall varies from 400 to 700
mm. The forested hills - no gauge- must receive over 1100 mm., a good example
of updating isohyetal maps by remotely sensed imagery. (see v.d. Laan, 1986;
Meijerink et aI., 1994). (4) The area with fairly thick colluvial and weathered zones
with low savannah vegetation, local recharge and stream recharge from the
adjoining hills. (5) Thin, young lavas resting on the basement, little recharge and
storage of groundwater.
Photo-lineaments can be described as; Linear structural elements which are
thought to have developed over fracture zones, and which are visible on remotely
sensed imagery. Some of them can be seen on the Colour Plate 14.B.
Interpreted lineaments can pertain to fractures of a different tectonic nature, with
or without intrusives or secondary clay fillings. Low topographic corridors of some
straightness, formed by denudation initiated by open fracturing, have been
identified as lineaments, as well as sharp, linear features in outcrop areas. There is
general agreement that the most promising water bearing directions originate from
brittle deformation caused by tensional stress related to normal faulting and strikeslip faulting (Larson, 1984; Greenbaum, 1 986; Castaing et al.,1989; Du Wenchai
and Ye Deliao, 1993). Different rocks respond to the same overall stress field with
different fracture densities. Grouping such rocks as gneisses which do not fracture
easily, together with granites which have the opposite behaviour, will therefore
cause much unnecessary variation in the statistical analysis.
The simple global relationship of well yield - distance to lineament may be of
little diagnostic significance. Indeed, the results of studies contradict each other
(see Waters, 1990, for a review and discussion). First of all, a segmentation
according to directions should be made, and specific capacity (l/hr/m.drawdown)
rather than yield (l/hr) should be used, if data permit.
The information of conventional rose diagrams can be supplemented, as Briere
and Razack (1982) have shown, by directional variograms and by relating the
results to the tectonic history of the area (e.g., Djeuda Tcapnga and Ekodek,
1990). Furthermore, the hydrogeologic situation around lineaments should be
considered, because much water may be derived from· weathered zones, with
possibly appreciable specific yields, adjacent to lineaments.
Because of their varied nature the hydrogeologic significance of a lineament
remains to be proven by drilling and well testing. (Carruthers et al. 1993).
In carbonate rocks (limestones, dolomites) lineaments are delineated in the
hopeful expectation that they represent zones of weakness extending sufficiently in
depth to have caused widening by solution by groundwater circulation. With the